The present invention is generally directed to aqueous dispersions of organic peroxides for use in curing polyester resins and the suspension polymerization of vinyl monomers. More particularly, the present invention is directed to aqueous dispersions of aromatic diacyl peroxides useful as initiators in the curing of unsaturated polyester resins and the suspension polymerization of vinyl monomers such as styrene, vinyl chloride, vinyl alcohol, vinyl acetate, methacrylates and acrylates, etc. In this regard, an important feature of the present invention is the provision of aqueous dispersions of aromatic diacyl peroxides which are pumpable and shelf-stable.
In the curing of polyester resins and the suspension polymerization of vinyl monomers, polymerization initiators are used extensively. Polyester resin compositions generally include mixtures comprising a monomer copolymerizable with an unsaturated organic polymer.
Aromatic diacyl peroxides are well known polymerization initiators. Peroxides have, however, as a general property, a tendency to be flammable and explosive with some peroxides exhibiting such properties to a greater extent than others. For example, benzoyl peroxide may decompose when dry due to shock, friction, or static electricity. This property carries with it the obvious hazards to the users of these materials as well as to the manufacturers and intermediate handlers thereof. One particularly burdensome aspect of this property occurs during shipment of the peroxides. Accordingly, it has long been an object to provide flame resistant organic peroxide compositions. For example, U.S. Pat. No. 3,507,800 is directed to providing a flame resistant peroxide composition consisting essentially of three components--water, peroxide and solvent wherein the water is at least about 18 percent of the composition.
The safety and end-use advantage provided by a water-soluble or a water-emulsifiable initiator is well recognized. In this regard, U.S. Pat. No. 3,825,509 describes a process for the suspension polymerization of vinyl chloride wherein the initiator is an aqueous emulsion of an organic peroxide in which the peroxide is present in an amount up to 19 weight percent. The surfactant used to prepare the aqueous peroxide emulsion is a combination of polyvinyl alcohol and polyoxyethylene sorbitan monolaurate. However, emulsions containing greater than about 19 percent by weight of organic peroxide are described as being too viscous and therefore difficult to handle.
Providing an aqueous emulsion of a highly reactive organic peroxide, in and of itself, is not a guarantee of a safe product, as phase separation can result in a concentrated mass of undiluted organic peroxide. Decomposition of such an undiluted mass of organic peroxide, depending on such things as the total mass, container, temperature of decomposition, etc., can result in a decomposition that is accompanied by the liberation of heat and flammable gases. Such conditions can lead to fires, deflagrations, pressure ruptures and detonations. Further, irrespective of any hazards, phase separation of the initiator emulsion can result in nonuniform concentrations of the initiator being supplied to the polymerization system. Efforts to provide a stable aqueous dispersion of organic peroxide include U.S. Pat. No. 3,988,261 which is directed to frozen aqueous emulsions of organic peroxides having a moderate to high concentration of organic peroxides. It has been suggested that such emulsions can be frozen without separation of the organic peroxide phase and water phase and, when thawed, retain their emulsified state at least for a period of time sufficient to allow safe handling and use. Although the frozen emulsified peroxide compositions of U.S. Pat. No. 3,988,261 may provide advantages for the handling of highly reactive peroxides, it is necessary for the end-user to provide special equipment for handling and thawing the frozen product. Consequently, it would be even more desirable to provide a pumpable product which can be shipped and forwarded directly from on-sight storage to the polymerization vessel.
Various compositions have been proposed for providing stable, pumpable, aqueous suspensions of organic peroxides. For instance, U.S. Pat. No. 4,039,475 discloses a pumpable aqueous suspension of organic peroxide solid at about 20.degree. C. containing a combination of at least 0.2 percent by weight of a non-ionic emulsifier having a maximum HLB-value of 12.5 and at least 0.2 percent by weight of a non-ionic emulsifier with an HLB-value not lower than 12.5 or at least 0.01 percent by weight of an anionic emulsifier as well as a water-soluble cellulose derivative as a thickener.
Another shelf-stable mobile free-radical generating initiator that can be readily pumped into a polymerization reactor and readily dispersed throughout an aqueous polymerization medium was described in U.S. Pat. No. 4,376,719 which discloses forming an aqueous dispersion by adding an initiator, such as benzoyl peroxide, to an aqueous system that contains a water-soluble suspending agent, such as water-soluble cellulose derivatives, and a wetting agent that may be non-ionic or anionic and homogenizing the suspension to form a stable initiator dispersion.
U.S. Pat. No. 4,415,716 proposes yet another composition for providing an aqueous dispersion of a free-radical forming initiator, which dispersion is stable and easy to handle. According to U.S. Pat. No. 4,415,716 such initiator dispersions are provided by an emulsifier system consisting of (a) an ethoxylated non-ionic emulsifier with an HLB-value above 15, which in most part does not contain cyclic inner ether bonds, and (b) a non-ethoxylated non-ionic emulsifier with an HLB-value below 9, whereby the resulting HLB-value of the emulsifier blend is within the range of 11-17.
Despite the numerous disclosures of organic peroxide dispersions useful as polymerization initiators, there is a need for improved aromatic diacyl peroxide dispersions that are stable and permit the initiator to be available in the polymerization mixture for initiating the curing of the unsaturated polyester or the suspension polymerization of vinyl monomers.
Accordingly, it is a general object of the present invention to provide an aqueous dispersion of a free-radical polymerization initiator or catalyst.
Another object of the present invention is to provide an improved aqueous dispersion of an aromatic diacyl peroxide polymerization initiator which dispersion is pumpable and easy to handle.
Another object of the present invention is to provide an aqueous dispersion of an aromatic diacyl peroxide polymerization initiator which is relatively shelf-stable.
Another object of the present invention is to provide a method for stabilizing a pumpable aqueous dispersion of an aromatic diacyl peroxide polymerization initiator useful for curing polyester resins.
Another object of the present invention is to provide a method for stabilizing a pumpable aqueous dispersion of an aromatic diacyl peroxide polymerization initiator useful for the suspension polymerization of vinyl monomers.
Another object of the present invention is to provide a method for curing unsaturated polyester resins.
Another object of the present invention is to provide a method for the suspension polymerization of vinyl monomers.
These and other objects of the present invention will be apparent from the following detailed description thereof.